WASHINGTON, July 30 (Reuters) - Mail industry companies areconcerned about a looming default by the U.S. Postal Service ona $5.5 billion payment for future retiree health benefits,saying it adds to uncertainty about agency and helps speed themovement away from traditional mail.

The Postal Service has said for months that it could notafford to make the massive payment, which was originally due in2011 but was delayed by Congress until Aug. 1.

Congress has so far made no significant push to delay thehealth pre-payment again. Missing the payment, the first defaultin the agency's history, would not cause interruptions inservice or prevent the Postal Service from paying suppliers andemployees, USPS spokesman David Partenheimer wrote in an email.

But the Mailing and Fulfillment Service Association said theapproaching default raises questions about the Postal Service'sfinancial stability and Congress's commitment to helping remedythe agency's money woes.

"Each one of these deadlines that gets hit and missed, Ithink, creates a degree of uncertainty in the minds of those whomake decisions about how they're going to manage specificmarketing campaigns," said Ken Garner, president of the grouprepresenting mailers, printers and other mailing industrybusinesses.

"It just creates again this movement away from mail as aproduct," he said.

The Postal Service, which lost $5.1 billion in fiscal year2011 and relies on sales of stamps and other products ratherthan taxpayer funds, wants congressional permission for businesschanges that include an end to Saturday delivery and an end tothe future retiree contributions.

The mail agency has said it also cannot afford a secondreitree health prefunding payment due at the end of the 2012fiscal year, in September, without legislative changes.

Even with the defaults, the agency could face a cashshortfall of $100 million in mid-October, the Postal Service'sinspector general wrote in a memo to Postmaster General PatrickDonahoe dated last week.

But the agency will likely win a short reprieve throughextra revenue from election-related mail that is expected tokick in about that time. Holiday mailing should also help getthe agency through the winter, but cash is expected to be tightagain starting in the spring of 2013, the memo said.

The USPS could save cash by suspending payments to aretirement account that already holds surplus funds, the reportsaid. The USPS has said in financial documents that in the eventof a shortfall, Congress likely would step in to keep the agencyopen.

As Americans have increasingly turned to onlinecommunication, the Postal Service has steadily lost money eachyear and has struggled to meet the annual, multi-billion-dollarretiree benefit payment obligations.

Lawmakers from both parties have said for more than a yearthat they planned to tackle legislation to overhaul the PostalService and help return it to profitability, but the House ofRepresentatives and the Senate do not appear close to a deal.

The Senate passed a bill in April that would spread theretiree health payments over more years, allow the PostalService to end Saturday mail delivery, and let it use surplusfunds in the retirement account to offer early retirementincentives as a way to reduce the workforce.

House leaders said last month they hoped to bring their ownbill - which would keep the health pre-payments and createoversight groups to close facilities and cut costs - up for avote before the summer recess. But a House vote has not beenscheduled, and lawmakers leave at the end of this week untilafter the U.S. Labor Day holiday in early September.

"It's well past time for House leaders to follow through onpast assurances to hold a vote on a postal reform bill," saidDemocratic Senator Tom Carper, one of the Senate bill's authors.

SHIFT TO ONLINE

Most members of the mailing community believe Congress willeventually pass postal legislation. But many said they worrythat the longer that takes, the more likely a postal overhaulcould involve painful changes in service - facility closings,delivery delays or rate increases.

Meanwhile, the uncertainty has spurred some businesses'efforts to look for ways to move transactions online, supplementdirect mail communications with email and social networking, orbranch out into new fields, industry insiders said.